<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>

  <form>
    <label for="age">年龄</label>
    <input id="age" type="number" placeholder="请输入年龄" value="20">
  </form>
</body>


</html>